Method of treating papermaking fibers for making tissue

ABSTRACT

The throughdryability of dewatered, but wet, sheets made from papermaking fibers can be significantly increased by subjecting an aqueous suspension of the fibers at high consistency to elevated temperatures with sufficient working of the fibers. Such a treatment is particularly effective for improving the efficiency of throughdrying processes used in the manufacture of certain products, such as tissues and paper towels, made from furnishes having a significant amount of secondary or recycled fibers.

This application is a continuation-in-part of U.S. patent applicationSer. No. 07/993,190 filed Dec. 18, 1992, now U.S. Pat. No. 5,348,620,which is a continuation-in-part of U.S. patent application Ser. No.07/870,648, filed Apr. 17, 1992, now abandoned.

BACKGROUND OF THE INVENTION

In the manufacture of certain paper sheet products, such as tissue andpaper towels, one method of drying the sheet after formation anddewatering is to pass heated air through the wet sheet in a process wellknown in the papermaking art as throughdrying. Throughdrying isadvantageous in that it minimizes compaction of the web and therebyproduces a bulkier product compared to conventional wet pressmanufacturing processes, which rely on high levels of compression of thewet web and on a Yankee dryer to dry the web. However, throughdrying hassome disadvantages in that it requires a substantial amount of expensiveequipment and energy to carry out the drying process. In particular, thedrying efficiency of the throughdrying process is in large partdependent upon the air permeability of the wet sheet which permits thehot air to pass therethrough. Air permeability is especially a problemfor sheets made from fiber furnishes containing secondary (recycled)fibers, which inherently impart poor air permeability to the wet sheetsinto which they are incorporated. With the continual efforts to utilizemore secondary fibers in paper products for environmental reasons, thereis a need to improve the throughdryability of secondary papermakingfibers.

SUMMARY OF THE INVENTION

It has been surprisingly discovered that the ease in which secondarypapermaking fibers can be throughdried can be improved by pre-treatingthe fibers of the papermaking furnish in a wet mechanical process inwhich the fibers are appropriately worked while suspended in a highconsistency aqueous slurry. The effectiveness of the pretreatment ismanifested by an increase of the Throughdryability Index (hereinafterdefined and referred to as the "TD Index") of the fibers. An increase inthe TD Index translates into faster throughdrying machine speeds for agiven furnish and basis weight, which results in improved operatingefficiency. The method of this invention can be utilized for anypapermaking fibers, but is especially advantageous for improving the TDIndex of secondary fiber furnishes and furnishes containing asignificant amount of fines and/or short fibers, such as hardwoodfibers. (As used herein, "short fibers" are fibers having a populationaverage length of from 0.5 to about 1 millimeter as determined by usinga Kajaani FS-200 fiber length analyzer.) In some cases, the TD Index ofsecondary fiber furnishes can be surprisingly improved beyond that ofuntreated virgin furnishes. As used herein, "fines" are particles thatpass through a 200 mesh screen having a 76 micrometer diameter opening.They can be measured in accordance with TAPPI Test Method T261 pm-80(1989).

In addition, while the fiber treatment of this invention is particularlyadvantageous for throughdrying processes and products, productimprovements can also be realized when the treated fibers of thisinvention are used for making wet pressed tissue products as well. Morespecifically, it has been found that substituting the treated fibers ofthis invention for a portion of the fibers of a given tissue furnish,the softness of the resulting tissue can be increased without loss ofstrength. This is especially effective when treating hardwood fibers andcombining the treated hardwood fibers with other fibers, such asuntreated softwood fibers, either blended or layered. The treatedhardwood fibers improve the softness of the resulting product while theuntreated softwood fibers retain the strength. Softness can be furtherenhanced by adding a softening agent to the treated fibers either beforetreatment or after treatment. Certain softening agents also provide anunexpected increase in bulk as well as enhancing the softness of thetissue.

Hence in one aspect, the invention resides in a method of making atissue comprising: (a) forming an aqueous suspension of papermakingfibers having a consistency (dry weight percent fibers in the aqueoussuspension) of about 20 or greater; (b) passing the aqueous suspensionthrough a shaft disperser at a temperature of 150° F. or greater with apower input of at least about 1 horsepower-day per ton of dry fiber,wherein the TD Index of the fibers is increased about 25 percent orgreater, preferably about 50 percent or greater, and more preferablyabout 75 percent or greater; (c) feeding the fibers through a tissuemaking headbox to form a wet web; and (d) drying the web, such as bythroughdrying, to form a dried tissue.

In another aspect, the invention resides in a method of making tissuecomprising: (a) forming an aqueous suspension of papermaking fibershaving a consistency of about 20 or greater; (b) passing the aqueoussuspension through a shaft disperser at a temperature of 150° F. orgreater with a power input of at least about 1 horsepower-day per ton ofdry fiber, wherein the TD Index of the fibers is about 0.15 or greater,preferably about 0.2 or greater, more preferably about 0.3 or greater,and most preferably about 0.5 or greater; (c) feeding the fibers througha tissue making headbox to form a wet web; and (d) drying the web, suchas by throughdrying, to form a dried tissue.

In a further aspect, the invention resides in a throughdried sheet madefrom a furnish comprising at least about 25 dry weight percent secondaryfibers, and/or having at least about 7 weight percent fines and/orhaving at least about 20 percent short fibers, said furnish having a TDIndex of about 0.15 or greater, preferably about 0.20 or greater, morepreferably about 0.30 or greater, and most preferably about 0.5 orgreater. If present, the amount of secondary fibers in the furnish canbe anywhere within the range of about 25 to about 50, 75, or 100 dryweight percent. In general, secondary fibers inherently have a highproportion of fines and/or short fibers. If present, the amount of finescan be about 7 weight percent or greater, more particularly about 10weight percent or greater, still more particularly from about 10 toabout 25 weight percent. If present, the amount of short fibers can beabout 20 weight percent or greater, more specifically about 50 weightpercent or greater, still more specifically about 75 percent or greater.

Papermaking fibers useful for purposes of this invention include anycellulosic fibers which are known to be useful for making paper,particularly those fibers useful for making relatively low densitytissue papers such as facial tissue, bath tissue, dinner napkins, papertowels, and the like. As used herein, the term "tissue" is usedgenerically and is intended to include all such products. Such productscan be creped or uncreped. The most common papermaking fibers includevirgin softwood and hardwood fibers, as well as secondary or recycledcellulosic fibers. As used herein, "secondary fiber" means anycellulosic fiber which has previously been isolated from its originalmatrix via physical, chemical or mechanical means and, further, has beenformed into a fiber web, dried to a moisture content of about 10 weightpercent or less and subsequently reisolated from its web matrix by somephysical, chemical, or mechanical means. Fibers which have been passedthrough a shaft disperser as described herein are sometimes referred toas "dispersed fibers".

The basis weight of the throughdried sheet can be from about 5 to about50, preferably from about 10 to about 40, and more preferably from about20 to about 30 grams per square meter. It will be appreciated that lowerbasis weight sheets inherently have greater permeability for a givenfurnish. Hence the greatest advantage of this invention is obtained withrelatively higher basis weights where the sheets are normally moredifficult to throughdry. The invention is particularly suitable formaking throughdried single-ply bath tissue having a basis weight ofabout 25 grams per square meter.

The consistency of the aqueous suspension which is subjected to thetreatment of this invention must be high enough to provide significantfiber-to-fiber contact or working which will alter the surfaceproperties of the treated fibers. Specifically, the consistency can beat least about 20, more preferably from about 20 to about 60, and mostpreferably from about 30 to about 50 dry weight percent. The consistencywill be primarily dictated by the kind of machine used to treat thefibers. For some rotating shaft dispersers, for example, there is a riskof plugging the machine at consistencies above about 40 dry weightpercent. For other types of shaft dispersers, such as the BIVIS shaftdisperser (commercially available from Clextral Company, Firminy Cedex,France), consistencies greater than 50 can be utilized without plugging.It is desirable to utilize a consistency which is as high as possiblefor the particular machine used.

The temperature of the fibrous suspension can preferably be about 150°F. or greater, more preferably about 210° F. or greater, and morepreferably about 220° F. or greater. In general, higher temperatures arebetter for increasing the TD Index. The upper limit on the temperatureis dictated by whether or not the disperser is pressurized, since theaqueous fibrous suspensions within apparatus operating at atmosphericpressure cannot be heated beyond the boiling point of water. A suitabletemperature range is from about 170° F. to about 220° F.

The amount of power applied to the fibrous suspension also impacts theresulting TD Index. In general, increasing the power input will increasethe TD Index. However, it has also been found that the improvement(increasing) of the TD Index falls off upon reaching a power input ofabout 2 horsepower-days per ton (HPD/T) of dry fiber in suspension. Apreferred range of power input is from about 1 to about 3 HPD/T, morepreferably about 2 HPD/T or greater.

In working the fibers within the disperser, such as by shearing andcompression, it is necessary that the fibers experience substantialfiber-to-fiber contact by rubbing or shearing in addition to rubbing orshearing contact with the surfaces of the mechanical devices used totreat the fibers. Some compression, which means pressing the fibers intothemselves, is also desireable to enhance or magnify the effect of therubbing or shearing of the fibers. The desired fiber-to-fiber contactcan in part be characterized by apparatus having a relatively highvolume-to-working surface area ratio which increases the likelihood offiber-to-fiber contact. The working surface for purposes herein isdefined as that surface of the apparatus which contacts the majority ofthe fibers passing through. For example, disc refiners have a very lowvolume-to-working surface area (approximately 0.05 centimeters) becausethere is a relatively small volume or space between the opposed rotatingdiscs (working surfaces). Such devices work the fibers primarily bycontact between the working surfaces and the fibers. However, theapparatus particularly useful for purposes of this invention, such asthe various types of shaft dispersers, have a much highervolume-to-working surface area. Such volume-to-working surface arearatios can be about 1 centimeter or greater, preferably about 3centimeters or greater, and more specifically from about 5 to about 10centimeters. These ratios are orders of magnitude greater than those ofdisc refiners.

The measure of the appropriate amount of shearing and compression to beused lies in the end result, which is the achievement of an increased TDIndex. A number of shaft dispersers or equivalent mechanical devicesknown in the papermaking industry can be used to achieve varying degreesof the desired results. Suitable shaft dispersers include, withoutlimitation, nonpressurized shaft dispersers, such as the Maule shaftdisperser, and pressurized shaft dispersers, such as the Bivis machinesand the like.

If softening agents are used to enhance the softness of the final tissueproduct, suitable agents include, without limitation, fatty acids,waxes, quaternary ammonium salts, dimethyl dihydrogenated tallowammonium chloride, quaternary ammonium methyl sulfate, carboxylatedpolyethylene, cocamide diethanol amine, coco betaine, sodium lauroylsarcosinate, partly ethoxylated quaternary ammonium salt, distearyldimethyl ammonium chloride, and the like. Examples of suitablecommercially available chemical softening agents include, withoutlimitation, Berocell 564 and 584 manufactured by Eka Nobel Inc., Adogen442 manufactured by Whitco Sherex Chemical Company, Quasoft 203manufactured by Quaker Chemical Company, and Arquad 2HT-75, manufacturedby Akzo Chemical Company.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a schematic flow diagram of the apparatus used for determiningthe TD Index.

FIG. 2 is an exploded perspective view of the sample holder of theapparatus of FIG. 1, including the sliding sample tray used to place thesample holder into the drying apparatus.

FIG. 3 is a representative depiction of a typical plot of pressure dropversus moisture ratio as generated while testing a sample using theapparatus described in FIG. 1 above.

FIG. 4 is a schematic process flow diagram of a method of treatingfibers in accordance with this invention using a shaft disperser to workthe fibers.

FIG. 5 is a cut-away perspective view of the shaft disperser of FIG. 4.

FIG. 6 is an alternative schematic process flow diagram of a method inaccordance with this invention using a pair of Bivis shaft dispersers inseries.

FIG. 7 is a sectional view of a Bivis shaft disperser useful forpurposes of this invention.

FIG. 8 is a sectional view, viewed in the axial direction, of thereverse-flighted screws of the Bivis shaft disperser, illustrating thecut-out notches in the flights.

FIG. 9 is a sectional view, viewed in the axial direction, of theforward-flighted screws.

FIG. 10 is a sectional view of a reversed-flighted section of themachine, illustrating the flow of the fibrous suspension.

THE THROUGHDRYABILITY INDEX

During throughdrying, it is generally understood that the drying rate ishigh and relatively constant at high moisture ratios (constant rateperiod). The drying rate begins decreasing rather rapidly after reachinga certain critical moisture ratio (falling rate period) in the sheet. Ifa constant air permeation rate is maintained throughout the dryingperiod, the pressure drop is also expected to decrease as the moistureratio decreases (or as the drying process continues). The manner inwhich the pressure drop varies during the throughdrying process under aconstant air permeation rate is of primary interest for purposes of thisinvention because it provides a quantitative means for measuring the airpermeability of the sheet while being dried. If one can accuratelymeasure the instantaneous absolute humidity of the outlet air afterdrying a tissue sample, one can readily calculate the instantaneousmoisture ratio from the humidity of the outlet air and the initial andthe final moisture ratios of the tissue sample as shown below: ##EQU1##wherein "X₀ "=the moisture ratio of the test sample at the beginning ofthe test, expressed as kilograms of water per kilogram of bone dryfiber;

"X_(end) "=the moisture ratio of the test sample at the end of the test,expressed as kilograms of water per kilogram of bone dry fiber;

"X_(m) "=the instantaneous moisture ratio of the test sample, expressedas kilograms of water per kilogram of bone dry fiber;

"Y_(in) "=the humidity of the drying air immediately prior to reachingthe sample, expressed as kilograms of water per kilogram of dry air;

"Y_(out) "=the humidity of the drying air immediately after passingthrough the sample, expressed as kilograms of water per kilogram of dryair; and

"t"=elapsed time, expressed in seconds.

Calculating the moisture ratio X_(m) from the humidity data for theentire drying period data enables one to plot pressure drop as afunction of the instantaneous moisture ratio. The inverse of the areaunder the resulting curve is referred to herein as the TD Index,expressed as kilopascals⁻¹. This index is a measure of the airpermeability of the wet sheet and reflects the ease in which a papersheet made from a particular furnish can be throughdried. Higher TDIndex values reflect greater ease in throughdrying, whereas lower valuesreflect greater difficulty in throughdrying.

As will be described below, measurement of the TD Index requires thatthe fibers in question be formed into a handsheet having a basis weightof 24 grams per square meter. This is accomplished by diluting a fibersample in water to a consistency of 2.5 weight percent in a British PulpDisintegrator and allowing the dispersed sample to soak for 5 minutes.The sample is then pulped for minutes at ambient temperature, diluted to0.04 percent consistency, and formed into a handsheet on a BritishHandsheet Mold (The Hermann Mfg. Co., Lancaster, Ohio). The handsheet iscouched off of the mold by hand using a blotter without applying anypressure. The handsheet is dried for 2 minutes to absolute dryness usinga Valley steam hotplate and a standard weighted canvas cover having alead filled (4.75 pounds) brass tube at one end to maintain uniformtension. The TD Index for the resulting dried handsheet is thendetermined as described herein and the measured TD Index value isassigned to the fibers or furnish from which the handsheet was made.

Referring now to FIG. 1, the apparatus for determining the TD Index willbe described in greater detail. Unless otherwise specified, conduit inthe mainstream of air flow has a 1.5 inch inside diameter. Air fordrying the samples is provided by two "oil free" compressors 1, eachrated for 29.9 cubic feet per minute at 90 psi. (Model DN 1024H-3DF,Atlas Copco, Cleveland, Ohio). The outlet of the compressors is suitablyconnected to the inlet of a condensed water separator 2 (ModelWSO-08-000, Wilkerson, Engelwood, Colo.), which serves to remove anyliquid water entrained in the air stream. The outlet of the separator issuitably connected to the inlet of a molecular sieve 3 (Model H530,Wilkerson) which serves to eliminate particulate matter in the airhaving a particle size greater than about 0.05 microns. The outlet ofthe molecular sieve is suitably connected to a compressed air dryer 4(Wilkerson model DHA-AE-000) with an outlet flow of 49 cubic feet perminute. The outlet of the dryer is connected to the inlet of a surgetank 5 (approximately 75 cubic feet capacity). The outlet of the surgetank is suitably connected to two additional oil heat exchangers 6 and 7(5 liter capacity/250° C. max. temp./6 bar max. pressure, ApparatebauWiesloch GmbH, Weisloch, Germany) in series which serve to further heatthe air to the desired throughdrying temperature. In between the surgetank and the two heat exchangers are a moisture monitor 8 (Aquanel,Gerhard GmbH, Blankebach, Germany), a gate-type control valve 9 (DINR65, 1.5 inch, PN 16, Henose, Hamburg, Germany) for controlling the flowrate of the air, an orifice plate 10 (25 millimeter diameter opening,RST 37-2 PN6 DIN 2527-32-5784, University of Karlsruhe, Karlsruhe,Germany), and a manometer 11 (Betz, Gottingen, Germany), which togetherare used to determine the air flow rate. Other valves and piping (notshown) which are not essential to the operation of the apparatus can bepresent for convenience at various places to isolate or by-pass gaugesand other devices. The outlet of the second heat exchanger is directlyconnected to a sample housing 12, which is designed to receive and holda slidable sample tray 13 (see FIG. 2) into which a sample holder (seeFIG. 2) is placed. All of the air passes through the sample placed inthe sample holder. An inflatable gasket mounted within the samplehousing assures a positive seal between the sample housing and theslidable sample tray when the gasket is activated. A diffuser 14 issuitably connected to the outlet of the sample housing such that thecross-sectional flow area is expanded to 11,600 square millimeters inorder to slow down the air flow to facilitate more accurate moisturemeasurement. The diffuser is suitably connected to a vent tube whichcarries the air to a suitable exhaust system. A differential temperaturesensor 16 (resistance type differentiation) is suitably connected tomeasure the temperature of the air before and after the sample. Adifferential pressure sensor 17 (PU 1000, 0-1000 millibar, 110 V AC) issuitably connected to measure the pressure drop across the sample. Asecond, more sensitive moisture monitor 18 (infrared; twenty-fivemeasurements per second; made by Paderborn University, Paderborn,Germany) measures the moisture content of the air leaving the sample.All three sensors are linked to a Schlumberger data acquisition system19 which is linked to a computer 20 (RMC 80286 processor) forcorrelating the data.

FIG. 2 illustrates the sample holder and the manner in which the sampleis mounted within the sample holder, including the sliding sample tray13 adapted to hold the sample holder and slide it in and out of thesample housing. Shown in FIG. 2 is the top of the sample holder 21,which is the upstream portion, and the bottom of the sample holder 22,which is the downstream portion. The bottom of the sample holdercontains a support fabric 23 (Asten 937, Asten Corporation, Appleton,Wis.) upon which the sample rests. Sandwiched in between the top andbottom is the handsheet sample 24 which has been cut to the appropriatesize. A thin line of grease (not visible in this view) positioned aroundthe inside edge of the top of the sample holder provides a seal betweenthe sample and the top of the sample holder when the sample is secured.The top of the sample holder contains two registration pins 25 whichbecome inserted into registration holes 26 in the bottom of the sampleholder and the two halves are secured by means of six screws 27 withappropriate threaded holes.

Having identified the apparatus for determining the TD Index, theprocedure for determining the TD Index can now be described. Generallyspeaking, to determine the TD Index for a given test sample (24 gsmhandsheet), the sample is carefully wetted to a particular moisturelevel and dried in the apparatus described above under controlledconditions of constant air mass flow rate. The moisture level in thesample is continuously calculated by a computer, based on the humiditymeasurements of the air before and after the sample during the test. Themeasured pressure drop across the sample is plotted as a function of thecalculated moisture level of the sample, and the area under the curve isthe TD Index.

More specifically, the handsheet sample to be tested is cut into a 10.25centimeter diameter circle which fits into the sample holder of theapparatus and is only slightly larger than the sample holder opening.During the test, only a 10 centimeter diameter circle of the sample isexposed to air flow. Accordingly, the portion of the circular sampleoutside the sample holder opening is impregnated with grease (Compound111 Valve Lubricant and Sealant, Dow Corning Corporation, Midland,Mich.) before being wetted for the test to prevent any moisture frombeing wicked outside the circle as well as establishing a more positiveseal between the sample and the sample holder. This is accomplished byapplying and rubbing the grease around the inside edge of the top half(upstream half) of the sample holder. The sample is then placed onto thebottom half of the sample holder which contains a piece of throughdryingfabric (Asten 937) for supporting the sample. The top half of the sampleholder is then clamped onto the bottom half of the sample holder,thereby impregnating the outer edge of the sample with the grease. Thesupported sample is then wetted by spraying with a fine mist until amoisture level of 3.0 kilograms of water per kilogram of bone dry fiberis reached. During spraying, a cover guard should be placed over thesample holder to prevent the sample holder from being sprayed and tokeep all of the sprayed water directed at the 10 centimeter circle. Themoisture in the sample is accurately determined by the weight differencebefore and after wetting.

While the sample is being prepared, the air system is turned on at apredetermined constant flow rate of 3.0 kilograms per square meter persecond and heated to a temperature of 175° C. When steady state isreached, the air temperature will be steady and constant, the humiditywill be zero, and the pressure drop will be zero. After the referencesteady state conditions are achieved, the sample holder with the wettedtest sample is placed into the sliding sample tray and slid into thesample housing of the apparatus. The humidity and the pressure drop arecontinuously monitored by the instruments and the computer, whichprovides a plot of pressure drop versus moisture ratio. A typical plotis illustrated in FIG. 3. (Note that time increases from right to leftin this plot.) As shown, the pressure drop shows a very rapid initialincrease and thereafter quickly starts decreasing, typically reaching aconstant level in about four or five seconds. The computer thenintegrates the inverse of the area under the curve and calculates the TDIndex as earlier described.

FIG. 4 is a block flow diagram illustrating overall process steps fortreating fibers in accordance with this invention. Shown is the paperfurnish 28 to be treated being fed to a high consistency pulper 29(Model ST6C-W, Bird Escher Wyss, Mansfield, Mass.) with the addition ofdilution water 30 to reach a consistency of about 15 percent. Prior tobeing pumped out of the pulper, the stock is diluted to a consistency ofabout 6 percent. The pulped fibers are fed to a scalping screen 31(Fiberizer Model FT-E, Bird Escher Wyss) with additional dilution waterin order to remove large contaminants. The input consistency to thescalping screen is about 4 percent. The rejects from the scalping screenare directed to waste disposal 32. The accepts from the scalping screenare fed to a high density cleaner 33 (Cyclone Model 7 inch size, BirdEscher Wyss) in order to remove heavy contaminants which have escapedthe scalping screen. The rejects from the high density cleaner aredirected to waste disposal. The accepts from the high density cleanerare fed to a fine screen 34A (Centrisorter Model 200, Bird Escher Wyss)to further remove smaller contaminants. Dilution water is added to thefine screen feed stream to achieve a feed consistency of about 2percent. Rejects from the fine screen are directed to a second finescreen 34B (Axiguard, Model 1, Bird Escher Wyss) to remove additionalcontaminants. The accepts are recycled to the feed stream to the finescreen 34A and the rejects are directed to waste disposal. The acceptsfrom the fine screen, with the addition of dilution water to reach aconsistency of about 1 percent, are then passed to a series of fourflotation cells 35, 36, 37 and 38 (Aerator Model CF1, Bird Escher Wyss)to remove ink particles and stickies. Rejects from each of the flotationcells are directed to waste disposal. The accepts from the lastflotation cell are fed to a washer 39 (Double Nip Thickener Model 100,Black Clawson Co., Middletown, Ohio) to remove very small ink particlesand increase the consistency to about 10 percent. Rejects from thewasher are directed to waste disposal. The accepts from the washer arefed to a belt press 40 (Arus-Andritz Belt Filter Press Model CPF 20inches, Andritz-Ruthner Inc., Arlington, Tex.) to reduce the watercontent to about 70 percent. Pressate is directed to waste disposal. Theresulting partially dewatered fibrous material is then fed to a shaftdisperser 41 (GR II, Ing. S. Maule & C. S.p.A., Torino, Italy),described in detail in FIG. 5, in order to work the fibers to increasethe TD Index in accordance with this invention. Steam 42 is added to thedisperser feed stream to elevate the temperature of the feed material.The resulting treated fibers 43 can be directly used as feedstock forpapermaking or otherwise further treated as desired.

FIG. 5 is a cut-away perspective view of a preferred apparatus fortreating fibers in accordance with this invention as illustrated in FIG.4. The particular apparatus is a shaft disperser, type GR II,manufactured by Ing. S. Maule & C. S.p.A., Torino, Italy. This apparatushas a volume-to-working surface area of about 8.5 centimeters. Shown isan upper cylindrical housing 51 and a lower cylindrical housing 52which, when closed, enclose a rotating shaft 53 having a multiplicity ofarms 54. The upper housing contains two rows of knurled fingers 55 andthree inspection ports 56. At one end of the upper housing is an inletport 57. At the inlet end of the rotating shaft is drive motor 58 forturning the shaft. At the outlet end of the rotating shaft is a bearinghousing 59 which supports the rotating shaft. The inlet end of therotating shaft contains a screw feed section 60 which is positioneddirectly below the inlet and serves to urge the feed material throughthe disperser. The outlet 61 of the disperser comprises a hinged flap 62having a lever 63 which, when the disperser is closed up, is engaged byhydraulic air bags 63 mounted on the upper housing. The air bags providecontrollable resistance to the rotation of the hinged flap and henceprovide a means of controlling the back pressure within the disperser.Increasing the back pressure increases the degree to which the fibersare worked and thereby increases the TD Index. During operation, theknurled fingers interdigitate with the arms of the rotating shaft towork the feed material therebetween.

FIG. 6 is a block flow diagram of an alternative process of thisinvention utilizing a pair of Bivis shaft dispersers. As illustrated,papermaking pulp, preferably secondary fiber pulp, at a consistency ofabout 50 percent, is fed to a screw feeder. The screw feeder meters thefeedstock to the first of two BIVIS shaft dispersers in series. EachBIVIS shaft disperser typically has three or four compression/expansionzones. Steam is injected into the first BIVIS shaft disperser to raisethe temperature of the fibers to about 212° F. The worked pulp istransferred to the second BIVIS shaft disperser operating at the sameconditions as the first disperser. The worked pulp from the secondmachine can be quenched by dropping it into a cold water bath andthereafter dewatered to a suitable consistency.

FIG. 7 is a sectional elevational view of a twin screw BIVIS shaftdisperser useful for purposes of this invention. Shown are the inlet 71,feed screw 72, forward flighted screws 73, 74, 75, and 76,reverse-flighted screws 77, 78, 79 and 80, outlet 81, injection ports 2,83, 84 and 85, optional extraction ports 86, 87, 88 and 89, andthermocouples 90. In operation, pulp is introduced into the BIVISthrough the inlet. The pulp then encounters the short feed screw, whichserves to introduce the pulp into the first working zone. The workingzones consist of a pair of slightly overlapping screws encased incylinders with less than 1 millimeter clearance between the screwflights and the cylinder walls. The twin screws rotate in the samedirection, and at the same speed. Shaft rotation transports the pulpaxially through the machine. Key to the fiber property modificationwithin the machine are the reverse-flighted screw sections which havesmall slots machined into the flights and are positioned periodicallyalong the length of both screws. These reverse-flighted sections serveto reverse the flow of stock through the machine, thereby introducingbackpressure to the stock stream. Thus the stock travels forward axiallyuntil it encounters a backpressure zone. The pressure builds in thiszone, but because of the slots in the reverse flights, the pressurebehind is greater than the pressure ahead. In this manner the stock isforced through the slots where it encounters the next (lower pressure)forward-flighted section of the screws. It is theorized that thiscompression/expansion action further enhances the modification of fiberproperties. Typically the BIVIS shaft disperser is set up to includethree or four working zones. The injection ports can be used to injectdifferent chemicals into each of the individual working zones. Theextraction ports associated with each working zone can be used toextract liquid if desired. Although not measured, the volume-to-surfacearea ratio of the BIVIS shaft disperser is believed to be slightly lessthan that of the Maule shaft disperser.

FIG. 8 is an axial view of a reverse-flighted section of the twin screwsof the apparatus of FIG. 7. Shown are screws 92 and 93, each havingslots 94 machined out of their flights. As shown, the flights of eachscrew overlap.

FIG. 9 is an axial view of a forward-flighted section of the twin screwsof the apparatus of FIG. 7, illustrating the overlap of the screwflights 95 and 96.

FIG. 10 is an expanded sectional view of a working zone of the apparatusof FIG. 7, showing the upstream forward-flighted screw section "A", thereverse-flighted screw section "B", and the downstream forward-flightedscrew section "C". This figure serves to illustrate the flow of stock(represented by the arrows) through the reverse-flighted screw section.

EXAMPLES

Having described the TD Index and the method of carrying out thisinvention, the invention will now be further described in detail withreference to the following examples.

Example 1

Secondary fiber (office waste grade) was treated by the processdescribed in FIG. 4. Specifically, the secondary fiber was repulped at aconsistency of 15%, cleaned, pressed to a pre-disperser consistency of22.9%, then fed to a Maule shaft disperser as illustrated in FIG. 5. Thefines content of the pulp fed to disperser was 8.5 weight percent. Thedisperser was maintained at a temperature of 175° F. Power input to thedisperser was approximately 1.39 horsepower-day per ton of fiber(HPD/T). Pre- and post-disperser samples were taken and made into dry 24gsm. handsheets per the previously defined method. Measurement of the TDIndex was carried out as previously described. The results are set forthin Table 1 below. (The "TD" is the Throughdryability Index, expressed askilopascals⁻¹. The "Tensile" is the tensile strength, expressed as gramsper inch of sample width. The "Tear" is the Elmendorf tear strength,expressed as grams-force per four sheets. The "Caliper" is thickness,expressed as inches. The "TEA" is the tensile energy absorbed, expressedas gram-centimeters per square centimeter.)

                  TABLE 1    ______________________________________    Sample   TD        Tensile Tear   Caliper                                            TEA    ______________________________________    Pre-disperser             0.11      1274    18.0   0.0058                                            8.1    Post-disperser             0.30      739     17.2   0.0076                                            3.8    ______________________________________

Example 2

The same secondary fiber material was treated as described in Example 1,except the disperser temperature was 175° F., the pre-disperserconsistency was 34.7 percent, and the power input to the disperser was2.12 HPD/T. Pre- and post-disperser samples were taken as before andformed into handsheets as previously described. The pre-disperser finescontent was 7.4 weight percent. The results are set forth in Table 2.

                  TABLE 2    ______________________________________           TD      Tensile Tear     Caliper                                          TEA    ______________________________________    Pre-disperser             0.12      1278    18.0   0.0060                                            6.6    Post-disperser             0.53      585     14.0   0.0082                                            2.5    ______________________________________

Example 3

The same secondary fiber material was treated as described in Example 1,except the disperser temperature was 150° F., the pre-disperserconsistency was 34.6%, and the power input to the disperser was 2.15HPD/T. Pre- and post-disperser samples were taken as before and formedinto handsheets as previously described. The results are set forth inTable 3.

                  TABLE 3    ______________________________________           TD      Tensile Tear     Caliper                                          TEA    ______________________________________    Pre-disperser             0.14      1334    19.2   .0060 7.9    Post-disperser             0.37      884     18.0   .0070 4.9    ______________________________________

Example 4

The same secondary fiber material was treated as described in Example 1,except the disperser temperature was 81° F., the pre-disperserconsistency was 26.8% and the power input to the disperser was 2.44HPD/T. Pre- and post-disperser samples were taken as before and formedinto handsheets as previously described. The pre-disperser fines contentwas 7.2 weight percent. The results are set forth in Table 4.

                  TABLE 4    ______________________________________           TD      Tensile Tear     Caliper                                          TEA    ______________________________________    Pre-disperser             0.10      1409    19.6   .0055 8.3    Post-disperser             0.17      1125    20.0   .0062 6.7    ______________________________________

Example 5

The same secondary fiber material was treated as described in Example 1,except the disperser temperature was 79° F., the pre-disperserconsistency was 34.6% and the power input to the disperser was 1.18HPD/T. Pre- and post-disperser samples were taken as before and formedinto handsheets as previously described. The pre-disperser fines contentwas 6.9 weight percent. The results are set forth in Table 5.

                  TABLE 5    ______________________________________           TD      Tensile Tear     Caliper                                          TEA    ______________________________________    Pre-disperser             0.10      1322    --     .0055 5.7    Post-disperser             0.14      1189    22.4   .0060 6.7    ______________________________________

Example 6

The same secondary fiber material was treated as described in Example 1,except the disperser temperature was 78° F., the pre-disperserconsistency was 37.1% and the power input to the disperser was 2.95HPD/T. Pre- and post-disperser samples were taken as before and formedinto handsheets as previously described. The pre-disperser fines contentwas 7.8 percent. The results are set forth in Table 6.

                  TABLE 6    ______________________________________           TD      Tensile Tear     Caliper                                          TEA    ______________________________________    Pre-disperser             0.10      1435    20.0   .0055 8.1    Post-disperser             0.19      1155    21.6   .0064 6.1    ______________________________________

Example 7

Well-washed secondary fiber at approximately 50% consistency was fed incrumb form to a BIVIS twin screw shaft disperser (Clextral Company,Firminy Cedex, France) via a screw feeder as illustrated in FIGS. 7-10.The fines content of the washed fiber was 5.2 weight percent. Steam wasintroduced into the interior compartment to raise the pulp temperatureto approximately 220° F. (105° C.). The product from the first BIVISdisperser was sent to a second BIVIS disperser operated under the sameconditions. From the second disperser, the pulp was sent to a quenchtank at 10° C. The pulp was then gravity dewatered and made into 24 gsmhandsheets as previously described and the TD Index was determined aspreviously described. The results are set forth in Table 7.

                  TABLE 7    ______________________________________            TD     Tensile Tear     Caliper                                          TEA    ______________________________________    Pre-BIVIS 0.08     1072    18.5   .0061 5.65    Post-BIVIS              0.71     388     10.5   .0079 1.28    ______________________________________

The results from the foregoing examples illustrate that theThroughdryability Index of secondary fibers can be dramaticallyincreased (and hence the throughdrying behavior of the fibers can beimproved) by subjecting the fibers to appropriate working forces at highconsistency and high temperature in a shaft disperser. Also, theapplication of high consistency working forces improves the tissuemaking character of the fibers by decreasing the tensile and tearingstrengths while increasing bulk.

Example 8

Cenibra eucalyptus fibers were pulped for 15 minutes at 10% consistencyand dewatered to 30% consistency. A softening agent (Berocell 584) wasadded to the pulp in the amount of 10 lb. Berocell per ton dry fiber,and the pulp was then fed to a Maule shaft disperser as illustrated inFIG. 5. The disperser was operated at 160° F. with a power input of 2.2HPD/T.

The resulting dispersed eucalyptus fibers were made into a two-layeredtissue having a softwood fiber layer and a eucalyptus fiber layer. Priorto formation, the northern kraft softwood fibers (LongLac-19) werepulped for 60 minutes at 4% consistency, while the dispersed eucalyptusfibers were pulped for 2 minutes at 4% consistency. Each layer wasindependently formed on separate forming fabrics at 0.05% consistency ata speed of about 50 feet per minute and the resulting two webs werecouched together at approximately 10% consistency to form a two-layeredweb. The resulting layered web was transferred to a papermaking felt andthereafter pressed onto the surface of a Yankee dryer, where the web wasdried and creped at a 1.3 crepe ratio. The dryer side of the layered webwas composed entirely of the softwood fibers and had a basis weight of7.25 lb./2880 ft² (dryer basis weight). The air side of the layered webwas composed entirely of the dispersed eucalyptus fibers of equal basisweight. After creping, the tissue was wound into bath rolls underminimum draw.

The resulting tissue had the following properties: tensile strength=858grams per 3 inches width (machine direction) and 488 grams per 3 incheswidth (cross-machine direction); stretch=30.4% (machine direction) and5.8% (cross-machine direction); Panel Softness=7.70. (Panel Softness isdetermined by a trained sensory panel which rates tissues for softnesson a scale of from 0 to about 9.5.) For comparison, a typical softnessvalue for throughdried material at similar strength is 7.15.

Example 9

Southern hardwood kraft fibers (Coosa River-59) were pulped for 15minutes at 10% consistency and dewatered to 28% consistency. A debonder(Berocell 584) was added to the pulp in the amount of 10 lb. Berocellper ton dry fiber. The pulp was then fed to a Maule shaft disperser. Thedisperser was operated at 160° F. with a power input of 2.2 HPD/T.

The resulting dispersed hardwood fibers were made into a two-layeredtissue having a softwood layer and a hardwood layer. Specifically,northern softwood kraft fibers (LongLac-19) were pulped for 60 minutesat 4% consistency, while the dispersed hardwood fibers were pulped for 2minutes at 4% consistency. Each layer was independently formed onseparate forming fabrics at 0.05% consistency and the resulting webswere couched together at about 50 feet per minute at approximately 10%consistency to form a single layered web. The resulting layered web wastransferred to a papermaking felt and thereafter pressed onto thesurface of a Yankee dryer, where the web was dried and creped at a 1.3crepe ratio. The dryer side of the layered web was composed entirely ofsoftwood fibers and had a basis weight of 7.25 lb./2880 ft² (dryer basisweight). The air side of the web was composed entirely of the dispersedhardwood fibers of equal basis weight. After creping, the tissue waswound into bath tissue rolls under minimum draw.

The resulting tissue had the following properties: tensile strength=689grams per 3 inches width (machine direction) and 466 grams per 3 incheswidth (cross-machine direction); stretch=32% (machine direction) and5.6% (cross-machine direction); Panel Softness=7.65. For comparison, atypical softness value for throughdried material at similar strength is7.30.

Examples 8 and 9 both illustrate the softness advantages of treatingvirgin hardwood fibers with a disperser in accordance with the method ofthis invention for making wet-pressed tissues.

Example 10

A secondary fiber blend (45 percent laser ledger, 45 percent lasercomputer print-out, 10 percent white ledger) was pulped for 15 minutesat 40° C. and 15 percent consistency. The resultant slurry was screenedat ambient temperature and 3.0 percent consistency. The slurry was thendehydrated to 30 percent consistency using a belt press and screw feddirectly to a shaft disperser (J. M. Voith GmbH, Heidenheim, Germany)similar to that illustrated in FIG. 5. The disperser was maintained at atemperature of 176° F. Power input to the disperser was approximately4.17 HPD/T. Pre- and post-disperser samples were taken and made into 24gsm handsheets per the previously defined method. As the improvement inTD Index occurs under all drying conditions, the TD Index of this samplewas measured under different flow conditions than the previous examples.Specifically, the air flow rate was 1.0 kg./m₂ sec., the air temperaturewas 90° C., and the initial moisture ratio was 3.5. Since each of thesefactors affects the TD Index, the TD Index values in this example arenot directly relatable to those of the previous examples. Still, theimprovement in the TD Index is readily apparent. The results are setforth in Table 8 below.

                  TABLE 8    ______________________________________    Sample            TD     Fines    ______________________________________    Pre-disperser     0.07   22.0    Post-disperser    0.18   23.3    ______________________________________

Example 11

The same secondary fiber blend was treated as described in Example 10,except the disperser was maintained at a temperature of 166° F. and thepower input to the disperser was approximately 5.28 HPD/T. Pre- andpost-disperser samples were taken and made into 24 gsm handsheets aspreviously described. Measurement of the TD Index was carried out as forExample 10. The results are set forth in Table 9 below.

                  TABLE 9    ______________________________________    Sample            TD     Fines    ______________________________________    Pre-disperser     0.07   22.0    Post-disperser    0.18   20.6    ______________________________________

Example 11

The same secondary fiber blend was treated as described in Example 10,except the disperser was maintained at a temperature of 147° F. and thepower input to the disperser was approximately 5.55 HPD/T. Pre- andpost-disperser samples were taken and made into 24 gsm handsheets aspreviously described. Measurement of the TD Index was carried out as forExample 10. The results are set forth in Table 10 below:

                  TABLE 10    ______________________________________    Sample            TD     Fines    ______________________________________    Pre-disperser     0.07   22.0    Post-disperser    0.17   20.6    ______________________________________

As illustrated by Examples 10, 11 and 12, the method of this inventiongreatly improves the TD Index even at very high fines levels.

It will be appreciated that the foregoing examples, given for purposesof illustration, are not to be construed as limiting the scope of thisinvention, which is defined by the following claims and all equivalentsthereto.

I claim:
 1. A method for making tissue comprising (a) forming an aqueoussuspension of papermaking fibers comprising at least 25 weight percentsecondary fibers and having a consistency of about 20 or greater; (b)passing the aqueous suspension through a shaft disperser having avolume-to-working surface area ratio of about 1 centimeter or greater ata temperature of 150° F. or greater with a power input of about 1horsepower-day per ton of dry fiber or greater, wherein the TD Index ofthe fibers is increased about 25 percent or greater; (c) feeding thefibers through a tissue making headbox to form a wet web; and (d) dryingthe web to form a dried tissue.
 2. The method of claim 1 wherein the TDIndex is increased about 50 percent or greater.
 3. The method of claim 1wherein the TD Index is increased about 75 percent or greater.
 4. Themethod of claim 1 wherein the temperature is about 210° F. or greater.5. The method of claim 1 wherein the temperature is about 220° F. orgreater.
 6. The method of claim 1 wherein the temperature is from about170° F. to about 220° F.
 7. The method of claim 1 wherein the amount ofsecondary fibers is about 50 weight percent or greater.
 8. The method ofclaim 1 wherein the amount of secondary fibers is about 75 weightpercent or greater.
 9. The method of claim 1 wherein the shaft disperserhas a volume-to-working surface area ratio of about 3 centimeters orgreater.
 10. The method of claim 1 wherein the shaft disperser has avolume-to-working surface area ratio of from about 5 to about 10centimeters.
 11. The method of claim 1 wherein the consistency is fromabout 20 to about 60 weight percent.
 12. The method of claim 1 whereinthe consistency is from about 30 to about 50 weight percent.
 13. Themethod of claim 1 wherein a softening agent is added to the papermakingfibers prior to passing the fibers through the shaft disperser.
 14. Themethod of claim 1 wherein a softening agent is added to the papermakingfibers while the fibers are passing through the shaft disperser.
 15. Themethod of claim 1 wherein a softening agent is added to the fibers afterthe fibers have passed through the shaft disperser.
 16. The method ofclaim 1 wherein the wet web is throughdried.
 17. A method for makingtissue comprising: (a) forming an aqueous suspension of papermakingfibers having a consistency of about 20 percent or greater, said fiberscontaining about 7 weight percent fines or greater; (b) passing theaqueous suspension through a shaft disperser at a temperature of 150° F.or greater with a power input of at least about 1 horsepower-day per tonof dry fiber, wherein the TD Index of the fibers is increased about 25percent or greater; (c) feeding the fibers through a tissue makingheadbox to form a wet web; and (d) drying the web to form a driedtissue.
 18. The method of claim 17 wherein the TD Index is increasedabout 50 percent or greater.
 19. The method of claim 17 wherein the TDIndex is increased about 75 percent or greater.
 20. The method of claim17 wherein the temperature is about 210° F. or greater.
 21. The methodof claim 17 wherein the temperature is about 220° F. or greater.
 22. Themethod of claim 17 wherein the temperature is from about 170° F. toabout 220° F.
 23. The method of claim 17 wherein the shaft disperser hasa volume-to-working surface area ratio of about 1 centimeter or greater.24. The method of claim 17 wherein the shaft disperser has avolume-to-working surface area ratio of about 3 centimeters or greater.25. The method of claim 17 wherein the shaft disperser has avolume-to-working surface area ratio of from about 5 to about 10centimeters.
 26. The method of claim 17 wherein the amount of fines isabout 10 weight percent or greater.
 27. The method of claim 17 whereinthe amount of fines is from about 10 to about 25 weight percent.
 28. Themethod of claim 17 wherein a softening agent is added to the fibersprior to passing the fibers through the shaft disperser.
 29. The methodof claim 17 wherein a softening agent is added to the fibers while thefibers are passing through the shaft disperser.
 30. The method of claim17 wherein a softening agent is added to the fibers after the fibershave passed through the shaft disperser.
 31. A method of making tissuecomprising (a) forming an aqueous suspension of papermaking fiberscomprising at least about 20 weight percent short fibers; (b) passingthe aqueous suspension through a shaft disperser having avolume-to-working surface area ratio of about 1 centimeter or greater ata temperature of 150° F. or greater with a power input of about 1horsepower-day per ton of dry fiber or greater, wherein the TD Index ofthe fibers is increased about 25 percent or greater; (c) feeding thefibers through a tissue making headbox to form a wet web; and (d) dryingthe web to form a dried tissue.
 32. The method of claim 31 wherein theTD Index is increased about 50 percent or greater.
 33. The method ofclaim 31 wherein the TD Index is increased about 75 percent or greater.34. The method of claim 31 wherein the temperature is about 210° F. orgreater.
 35. The method of claim 31 wherein the temperature is fromabout 170° F. to about 220° F.
 36. The method of claim 31 wherein theamount of short fibers is about 50 percent or greater.
 37. The method ofclaim 31 wherein the amount of short fibers is about 75 percent orgreater.
 38. The method of claim 31 wherein the short fibers are virginhardwood fibers.
 39. The method of claim 31 wherein the shaft disperserhas a volume-to-working surface area ratio of about 3 centimeters orgreater.
 40. The method of claim 31 wherein the shaft disperser has avolume-to-working surface area ratio of from about 5 to about 10centimeters.